1. Technical Field
The present invention relates to colonography, and more particularly, to a system and method for polyp detection in tagged or non-tagged stool images.
2. Discussion of the Related Art
Early detection of colon cancer through polyp screening via computed tomography colonography (CTC) is gaining increasing acceptance. CTC involves two high-resolution CT scans taken at prone and supine positions. However, current prone and supine acquisition datasets can be time consuming for physicians such as radiologists to analyze. Recently, computer aided detection (CAD) methods have been introduced to help radiologists find polyps in such datasets. These methods have also been shown to work with other scanning modalities such as magnetic resonance imaging (MRI).
Although CAD methods promise to help reduce polyps that are missed by radiologists, patient discomfort remains high since they must still undergo the standard colonography preparation regimen. To make the examination process more comfortable, radio-opaque liquids can be given to the patient a few days before the scan, instead of a more restrictive liquid-only diet followed by laxatives to clean out the colon. By using radio-opaque liquids, patient discomfort is reduced at the cost of more residual stool within the colon. However, this stool is tagged by the liquids and is clearly distinguishable from other elements within the body.
Frequently, despite a cleansing regimen, residual stool will remain in the colon and can lead to false positives for both radiologists and CAD methods. Tagged stool, on the other hand, appears very bright within the images and can be easily identified. However, even with tagged stool, it can still be difficult for radiologists and CAD methods to detect polyps. For example, the bright stool can be distractive and create artifacts in the images. Further, standard CAD methods are not designed to handle tagged stool images, thereby leading to errors such as false positives when a polyp is covered or partially covered by tagged stool.
Current research focuses on using tagged stool subtraction techniques to correct these errors. Examples of such research are presented in Zalis M, Yoshida H, Nappi J, Magee C, and Hahn P, “Evaluation of False Positive Detections in Combined Computer Aided Polyp Detection and Minimal Preparation/Digital Subtraction CT Colonography (CTC)”, RSNA 2004, and Macari M, Mikheev A, Neil J, Lee J, and Rusinek H, “Fully Automated Segmentation of Tagged Fecal Material: Validation of CT Colonography of the Unprepped Colon”, RSNA 2004.
These approaches attempt to subtract stool from an image of a colon while retaining the smooth borders of the colon wall and preserving any underlying polyps. However, these approaches are not yet fully developed and may still create artifacts. artificial borders and rough edges, which in turn can cause errors for radiologists and existing CAD methods. Although existing CAD methods have been applied directly to tagged stool datasets as described, e.g., in H Yoshida, P Lefere, J Nappi, S Gryspeerdt, A Okamura, and A Dachman, “Computer-aided Detection of Polyps in CT Colonography with Dietary Fecal Tagging: Pilot Assessment of Performance”, RSNA 2004, the majority, if not all, of the polyps detected were not covered by or even near tagged stool.